Synaptic vesicles are the characteristic morphological features of nerve terminals. They contain neurotransmitter which is released during electrical stimulation presumably by an exocytotic mechanism. Synaptic vesicles from the electric organ of Narcine brasiliensis contain antigens shared by no other major subcellular element. Antibodies to these antigens therefore allow vesicles to be recognized both biochemically and anatomically. Using both these procedures, antibody binding will be used to follow the fate of synaptic vesicle antigens during exocytosis and the membrane recycling events which follow exocytosis. Characterization of the phospholipid, glycolipid and protein composition of the pure vesicle will support or eliminate several of the models of exocytosis presently in vogue, and may suggest some new ones.